Is Cobalt in Li-Rich Layered Oxides for Li-Ion Batteries Necessary?

Cobalt is considered an essential element for layered cathode active materials supporting enhanced lithium-ion conductivity and structural stability. Herein, we investigated the influence of Co concentration on the physicochemical properties and electrochemical performance of lithium-rich layered oxides (LRLOs) with different Co content (Li_1.2Ni_0.2-x/2Mn_0.6-x/2Co_xO₂, x=0, 0.04, and 0.08). Though the presence of Co grants structural stability to LRLOs, superior long-term cycling stability is achieved with the Co-free LRLO retaining 88.1 % of the initial specific capacity (vs. 75.9 % of Li_1.2Ni_0.16Mn_0.56Co_0.08O₂) after 300 galvanostatic cycles at 250 mA g⁻¹ (1 C). The chemical stability on the surface of LRLOs containing Co declines faster, indicating a higher bulk structural stability not being the primary determinant of the LRLOs’ cycling performance. Ex-situ investigations indicate that the superior cycling stability of Co-free LRLO is obtained by reducing the Mn-related redox at discharge, which contributes to the large degree of polarization and low energy efficiency. Finally, the full-cell configured with the optimized LRLO as cathode and graphite anode delivers an energy density of 464 Wh kg⁻¹ at C/10, and 74.4 % and 94.3 % of retention in discharge specific capacity and average voltage at the 1000^th cycle, demonstrating the applicability of Co-free LRLO for sustainable LIBs.